1. Field of the Invention
The present invention relates to a method of forming a volume type phase hologram. More particularly, the present invention relates to a method of stably forming a uniformly developed volume type phase hologram having an increased diffraction efficiency over a large area by using a monobath or single step development process in which a mixture of a good solvent with a lower boiling point and a poor solvent with a higher boiling point is utilized. The method of the present invention can be used in the production of recording materials, optical elements and other devices.
The present invention also relates to an apparatus for forming a volume type phase hologram, particularly a developing apparatus thereof.
2. Description of the Related Art
As is well known in the art, holography is a photographic process and according to this process in which a subject to be recorded is irradiated with a good coherent radiation such as a laser beam, the radiation amplitude and phase is modulated according to the shape of the subject, and then interference fringes of radiation reflected on or transmitted through the subject is recorded in a holographic material to form a hologram producing an optical image of the subject. Further, the optical image can be reproduced by again irradiating the hologram with the radiation. Recently, hologram have been utilized in the production of high-function and compact optical elements (HOE; Holographic Optical Element), because various functions of the optical element can be concentrated in a thin layer of the hologram, in addition to the utilization of holograms as a recording material. Typical examples of the optical elements include a reading system for bar code readers and a laser scanning system for laser printers.
Hitherto, the formation of the hologram has been carried out in a two-step development process. Namely, as illustrated in FIG. 1, the prior art formation of the hologram comprises the steps of:
exposing a holographic material to an interference pattern of radiation,
swelling the pattern-wise exposed holographic material in a first solvent as a swelling solution, and
shrinking the swollen holographic material in a second solvent as a shrinking solution. The thus obtained hologram has excellent properties such as a high diffraction efficiency and good stability. The two-step development process and the formation of the hologram using this process can be found in many patent disclosures, for example:
(1) Japanese Unexamined Patent Publication (Kokai) No. 53-15152, published on Feb. 10, 1989, and the corresponding U.S. Pat. No. 4,173,474; PA0 (2) Japanese Unexamined Patent Publication (Kokai) No. 53-15153, published on Feb. 10, 1989, and the corresponding U.S. Pat. No. 4,172,724, No. 4,258,111 and No. 4,287,277; PA0 (3) Japanese Unexamined Patent Publication (Kokai) No. 54-101343, published on Aug. 9, 1979, and the corresponding U.S. Pat. No. 4,201,441; PA0 (4) Japanese Unexamined Patent Publication (Kokai) No. 54-102140, published on Aug. 11, 1979, and the corresponding U.S. Pat. No. 4,201,441; PA0 (5) Japanese Unexamined Patent Publication (Kokai) No. 63-254485, published on Oct. 21, 1988; and PA0 (6) Japanese Unexamined Patent Publication (Kokai) No. 63-266478, published on Nov. 2, 1988. Note, among these patent publications, the publications (5) and (6) above were published in Japan after the filing date of Japanese Patent Application No. 63-252501, filed on Oct. 6, 1989, which is a basic application of the present application.
Among the above publications, for example, Japanese Kokai No. 53-15153 concerns a hologram produced by causing a crosslinking reaction in accordance with an interference pattern in a recording carrier composed of a water-insoluble polymer containing in the unit structure thereof an aromatic or heterocyclic ring having a reactive site which can be replaced by a radical, and a halogen-containing compound. The water-insoluble polymer is preferably polyvinyl carbazole, and the halogen-containing compound is preferably a polyhalogen compound. Using this recording carrier, the hologram can be formed as follows: After exposure of the recording carrier to an interference pattern of radiation, the exposed recording carrier is first dipped in a first solvent having a swellability, i.e., a swelling solution to cause the carrier to swell and at the same time, to remove unreacted halogen-containing compound from the carrier. The swelling of the carrier, which corresponds to the latent hologram pattern formed in the previous exposure step, occurs throughout the overall thickness of the carrier. After completion of the swelling treatment, the swollen recording carrier is removed from the swelling solution, and then dipped in a second solvent which is a poor solvent for the recording carrier and is compatible with the first solvent, i.e., a shrinking solution. When the carrier is immersed in the shrinking solution, shrinkage of the carrier immediately occurs as a result of replacement of the first solvent with the second solvent, and thus a differential refractive index between the exposed and unexposed areas or a distribution of the refractive index is produced in the carrier. This distribution of the refractive index relies upon the formation of microporous voids caused by a separation of the solvents during the solvent substitution. Compared with the weakly exposed areas, the strongly exposed areas will produce less voids because of they are difficult to swell, and thus show a relatively increased refractive index. Upon completion of these swelling and shrinking steps, the hologram is fixedly produced in accordance with the swollen state of the carrier.
The two-step development processes described in Japanese Kokai No. 53-15153 and the related literatures listed above have problems in common. Namely, the problem caused by swelling solution remaining on the swollen carrier, just before the shrinking treatment. The inventors found that the amount of remaining swelling solution can have a remarkable effect on the optical properties of the resultant hologram, in addition to the degree of the swelling of the carrier and dissolution of the water-insoluble polymer from the carrier in the swelling solution. More particularly, if the amount of the swelling solution adhered to the carrier surface is unacceptably high, excess amounts of the swelling solution remain as droplets, which will remain as traces in the resultant hologram. Further, if lesser amounts of the swelling solution are used, a desired diffraction efficiency cannot be obtained due to the reduced swelling level and subsequent weak development.
Another problem is that of the time lag from the end of the swelling treatment to the start of the shrinking treatment. The inventors found that the swollen recording carrier, after removal from the swelling solution and before immersion in the shrinking solution, can be adversely affected by atmospheric conditions such as wind and temperature, i.e., the vaporization of the adhered swelling solution can occur, and accordingly the resultant holograms do not have uniform properties.
A third problem is that of the differential exposure of the pulling swollen carrier to the atmosphere. The inventors found that, when the swollen carrier is vertically pulled up from the swelling solution, a notable differential exposure of the carrier to the atmosphere occurs due to of the length of the carrier. Apparently, an upper portion of the pulling carrier is exposed to the atmosphere for a relatively long time, compared with the time of exposure of a lower portion of the carrier, and due to this longer exposure time, the development of the upper portion is weaker than that of the lower portion. The problem of the differences of the development strength is more severe for a recording carrier in which polystyrene or polyvinyl carbazole is used as the matrix polymer, in comparison with a recording carrier in which polyvinyl alcohol or polyvinyl pyrrolidone is used as the matrix polymer. This is because the former requires use of the volatile organic solvents as the good solvent, and the latter requires use of the substantially non-volatilizable solvents such as water, as the good solvent. Further, although such longitudinal differences of the development strength are negligible for small-sized holograms, they are particularly remarkable and serious when large-sized holograms are produced. It should be noted that, with regard to this third problem, although the value of the differences may be more or less varied, it essentially occurs in the two-step development process in which the recording carrier is removed from the developer in the course of the development and exposed to the atmosphere such as air or nitrogen gas, and is immersed again in the same or in a different developer to complete the development process. Therefore, to produce large-size and uniform holograms, there is a need for an improved development process in which the above problems are eliminated.